1. Field of the Invention
The present invention relates to pressure sensitive adhesives which provide suitable adherence of objects to the human skin. The invention relates more particularly to silicone based pressure sensitive adhesives which are of particular use in transdermal therapeutic devices for attachment to the human skin.
2. Description of the Prior Art
Pressure sensitive adhesives for use on human skin are used typically in bandages or other therapeutic devices which must adhere to the skin for a prescribed period of time. Such devices are typically comprised of a plastic or cloth film layer coated with a pressure sensitive adhesive. The pressure sensitive adhesive is protected with a release substrate which is readily peelable from the adhesive coating. The pressure sensitive adhesive for use in connection with such bandages or other therapeutic device must satisfy an array of specific physical characteristics. Importantly the release substrate must be easily peelable from the adhesive coating and the adhesive must have sufficient cohesion to keep the bandage or therapeutic object in adhesive contact with the skin for a prescribed period of time. Also the adhesive must not cause skin inflammation and must be nontoxic.
Additionally a pressure sensitive adhesive as applied to a transdermal therapeutic device must meet other strict performance requirements. A transdermal device is a medicinal pouch which contains a liquid medicine or other drugs which must be absorbed gradually into the skin over a fairly long period of time. These devices typically contain a semipermeable membrane and are advantageously used with drugs which are best absorbed into the human body gradually, such as nitroglycerine or other drugs useful in treating cardiac impairment. Thus, the pressure sensitive adhesive which is coated onto the therapeutic device must not act as a barrier to interrupt the flow of a fluid from the device and into the human blood stream. Specifically, the pressure sensitive adhesive must be permeable to the drug being used. Furthermore, since the therapeutic device must remain in close adhesive contact with the skin over a period of time typically at least up to 24 hours, the adhesive should keep essentially all of the contact surface of the therapeutic device in adhesive contact with the patient's skin over this entire period. The pressure sensitive adhesive should also permit the therapeutic device to be peeled from the skin without causing discomfort and without leaving an adhesive residue on the skin which is not easily removeable.
U.S. Pat. Nos. 2,857,356 and 4,039,707 are illustrative of prior art silicone-based pressure sensitive adhesives. U.S. Pat. No. 2,857,356 discloses a pressure sensitive adhesive formed from the polymerization of a silicate resin and an organopolysiloxane fluid. The silicate resin is obtained by intercondensing a mixture comprising a cohydrolysis product of a trialkyl hydrolyzable silane and an alkyl silicate, said cohydrolysis product containing a plurality of silicone-bonded hydroxy groups. The formulation disclosed in this reference is directed principally to forming a pressure sensitive adhesive which retains a high degree of tack and cohesion over a wide temperature range, for example between about -75.degree. C. and 250.degree. C. The pressure sensitive adhesive product disclosed in this reference is directed to application principally as an adhesive coating for use in contacting polymeric material such as glass, a wide range of plastics such as polyethylyene, and also for use in connection with the manufacture of pressure sensitive tapes. There is no reference made in this disclosure to suitability of the adhesive product for application to human skin. In order to achieve the high degree of tack and cohesive strength, patentees disclose that the weight ratio of the silicate resin to the organopolysiloxane fluid should be between about 0.5/1 to 6/1, more preferably between about 1/1 to 3/1. (Column 7, lines 21 to 28). The range of formulations disclosed in this reference could not be suitable for application to a transdermal therapeutic device because of insufficient tack for application to human skin and too great a time lapse required to achieve maximum bonding.
U.S. Pat. No. 4,039,707 discloses a siloxane-type pressure sensitive adhesive which is composed of the intercondensation product of a mixture containing an organopolysiloxane resin and at least one alkylaryl polysiloxane gum. The organopolysiloxane resin is defined at Column 6, lines 5 to 10 as having the formula: EQU (OH).sub.q (OR.sup.2).sub.r R.sup.2 SiO.sub.4qrs/2
It is also disclosed as essential that the organopolysiloxane gum contain aryl groups, such as phenyl groups, to obtain optimum pressure sensitive adhesive properties. If a mixture of phenyl and methyl groups is employed, this reference teaches that the number of silicone-bonded phenyl groups should be maintained with such a range that for each 2 to 75 phenyl groups attached directly to silicone by a carbon silicone linkage there are present from 98 to 25 silicone-bonded methyl groups, with the preferred range for instance from about 5 to 15 phenyl groups per 95 to 85 methyl groups. (Column 5, line 59 to Column 6, line 2). This reference also discloses a use of a release substrate covering the pressure sensitive adhesive. The release substrate may typically be a paper or polymer film coated with a release coating such as a silanol-stopped dimethylpolysiloxane fluid. Another release coating which is disclosed in this reference is based on dimethylvinyl-stopped dimethyl polysiloxane fluid. Additionally, the reference discloses that the alkylaryl polysiloxane gum should have a viscosity from about 200,000 to 15 million centipoise at 25.degree. C. and contain an average from about 1.85 to 2.01 silicone-bonded alkyl and aryl radicals per silicone atom.
Although use of an alkylaryl polysiloxane gum with an organopolysiloxane resin is disclosed in this reference, the reference does not disclose or suggest use of specific combinations of alkylaryl polysiloxane gums to improve the product's adhesive, shear and liquid permeability characteristics so that the adhesive product may be used with transdermal therapeutic devices.
Accordingly, it is an object of the present invention to provide an improved pressure sensitive adhesive which has particular use as an adhesive for retaining a plastic film or therapeutic device in adhesive contact with the human skin and which adhesive is nontoxic and easily removeable from the skin.
An important object of the invention is to provide a pressure sensitive adhesive for particular use in transdermal therapeutic devices. A related object is to provide pressure sensitive adhesive which retains the therapeutic device in adhesive contact with the human skin over a prolonged period of time and does not interfere with the absorption of medicinal fluid from the transdermal device through the adhesive and into the skin.